The number and variety of commercially available display devices have greatly increased in recent years. Computers (whether desktop, laptop, or notebook), personal digital assistants (PDAs), mobile phones, miniature music players, and thin LCD TVs are but a few examples. Although some of these devices can use ordinary ambient light to view the display, most include an extended area light source, referred to as a backlight, to make the display visible.
Many such backlights fall into the categories of “edge-lit” or “direct-lit”. These categories differ in the placement of the light sources relative to the output area or face of the backlight, where the output face defines the viewable area of the display device. In edge-lit backlights, a light source is disposed along an outer border of the backlight construction, outside the area or zone corresponding to the output face. The light source typically emits light into a light guide, which has length and width dimensions on the order of the output face and from which light is extracted by extractor features to illuminate the output face. In direct-lit backlights, an array of light sources is disposed directly behind the output face, and a diffuser is placed in front of the light sources to provide a more uniform light output. Some direct-lit backlights also incorporate an edge-mounted light, and are thus capable of both direct-lit and edge-lit operation.
Backlights typically incorporate a variety of light management films to help achieve certain design goals. The design goals may include backlight brightness, efficiency, spatial uniformity over the area of the output face, polarization requirements (e.g. whether polarized light output is necessary or desirable), as well as cost, complexity, size, thickness (thinness), ease of manufacture or assembly, and/or combinations thereof. One key performance metric for liquid crystal display (LCD) backlights is the brightness measured along the direction perpendicular to the plane of the display, referred to as the axial luminance.
Examples of light management films used to achieve these design goals are prism films, diffuser films, polarizing films, mirror films, retarder films, and the like. Many of these film types can be classified further. For example, some prism films, referred to as “turning films”, are arranged in edge-lit backlights with prism peaks that are typically oriented toward the light guide. Light exits the light guide and encounters the prism structures. The light is refracted by a first facet of the prism structures and is reflected by a second facet of the prism structures so that it is redirected or “turned” toward the optical axis or viewing axis of the backlight. Another type of prism film “recycles” a portion of the light to increase the amount of on-axis light exiting the display. These prismatic recycling films, also known as brightness enhancement films or “BEF” films, include a prism film layer that has prism peaks that are typically oriented away from the light guide. The prism peaks may be rounded or truncated to achieve particular optical characteristics. As light from the light guide passes through the film and encounters the prisms, a portion of the light is refracted in the on-axis direction, while another portion of the light is reflected by total internal reflection back toward the light guide. The reflected light may be recycled until it eventually emerges from the display.
Many backlights incorporate two prismatic recycling films that are “crossed”, i.e., rotated 90 degrees relative to each other, so as to achieve brightness enhancement in two orthogonal viewing planes. At least in the case of edge-lit backlights, a separate diffuser film is also typically included between the light guide and the crossed recycling films. The diffuser film employs scattering elements to help redirect light emitted by the light guide, which is typically highly oblique, into a wider angular range whose maximum luminance is closer to the axial direction. The crossed recycling films then redirect some of this wider angle light back towards the diffuser film and light guide for recycling, and redirect some of the light in a forward direction to enhance the axial luminance. It is standard industry practice to include the diffuser film between the light guide and the crossed recycling films; industry backlight designers consider it necessary to use such a diffuser film in combination with the crossed recycling films to maximize the axial luminance of the backlight.